1. Field of the invention
The present invention concerns generally a container for culture mediums. More particularly, this invention relates to a container adapted to receive one or more culture mediums for micro-organisms, a method of filling this device with such a medium or mediums and the use of such a container for the detection of micro-organisms with a view to identifying and counting them.
2 . Description of the prior art
In many industries, such as the foodstuffs, pharmaceutical and electronics industries, and in hospitals, it is essential to be able to evaluate the degree to which certain products or surfaces are contaminated by micro-organisms such as bacteria, yeast and mold.
To carry our this evaluation in the case of a liquid product a sample is usually filtered through a sterile membrane filter, the membrane filter is placed on a gel nutrient culture medium in a container. This container is then incubated at an elevated temperature for a time sufficient to enable any micro-organisms that may be present in the sample to grow and multiply in the form of colonies visible to the naked eye to permit counting and identification.
One operation that is important to correctly implement this technique for counting micro-organisms on a microporous membrane filter is the placing of the filter used to separate the micro-organisms in contact with the gel culture medium. Thus the condition of the surface of the gel culture medium is an important factor in the success of the operation.
There currently exist containers referred to as Petri dishes which are made from glass or plastic. Petri dishes are usually supplied in a sterile condition for single use and are made in two parts, namely a lid and a base into which the gel nutrient medium is poured after it has been heated to liquefy it. Thereupon, the lid is merely placed over the base to cover the medium.
After the gel medium has cooled and solidified, the Petri dishes may be used or stored in a refrigerator to obtain the optimum storage conditions. The surface of the gel culture medium is then brought into contact with the microporous filter on whose top surface any micro-organisms will have been collected. Growth of the micro-organisms is then dependent on capillary action allowing media to pass through the pores of the membrane.
The use of Petri dishes has a number of disadvantages. Firstly, the surface of the gel medium may feature defects due either to an inadequate casting temperature or to excessively sudden cooling of the medium. The resulting so-called "orange peel" effect results in surface defects which compromise good contact between the culture medium and the bottom surface of the membrane filter. Good contact is essential for the micro-organisms to develop.
Also, the surface of the gel medium is deformed (i.e., not flat) at its edge because the media tends to rise by capillary action up the vertical walls of the base of the Petri dish to form a fillet. This limits the surface area of the gel that can be used and, to compensate, it is common practice to employ membrane filters with a diameter substantially less than that of the Petri dish. This results in an increase in overall size of the device and a significant volume of gel that is not used.
When the Petri dish is being filled with the gel medium there is also the risk that air bubbles may burst on the surface and, because of the viscous nature of the medium, produce craters which also compromise good contact with the membrane filter.
Because Petri dishes are filled at a raised temperature, as the culture medium cools after the lid is placed on the device, condensation occurs on the bottom of the lid. The condensate sometimes runs down onto the top of the gel medium, thereby requiring a drying operation in an oven before use. This phenomenon is accentuated when the dishes are stored in a refrigerator. Allowing the culture medium to cool without the lid exposes the surface of the gel culture medium to risks of exogenous contamination when it is stored in unsealed Petri dishes.
Finally, because of the concave shape of the surface of the medium, Petri dishes do not enable direct effective counting of any micro-organisms present on the surface by simple contact between the surface of the gel medium and the bottom surface of the membrane.
U.S. Pat. No. 4,326,028 discloses an improved Petri dish comprising a cylindrical casing to which is attached a perforated support grid and which is designed to receive in back-to-back relationship two different culture mediums. This arrangement enables observation in two compartments situated one on each side of the grid; however, the surfaces of both of the culture mediums are subject to the same disadvantages as those that affect Petri dishes as described above.
An object of the present invention is an improved container for the same application as Petri dishes but without he disadvantages mentioned above.